Recently, photolithography technology widely adopted in an exposure apparatus for fabricating a semiconductor device such as VLSI. The exposure apparatus is provided with a mask having many apertures composing a predetermined pattern, a projection lens positioned under the mask, and a wafer positioned under the projection lens.
In the exposure apparatus, when coherent illumination is transmitted through the mask and the projection lens, the predetermined pattern is printed on a surface of the wafer to be scale-downed. According to the exposure apparatus, however, there is a problem in that resolution of the printed pattern is decreased because of interference effect, if the mask has several adjacent apertures.
Accordingly, a phase shifting mask using a phase shifter is proposed to solve the above problem.
A conventional phase shifting mask is provided with two adjacent apertures in which one of the apertures is covered with a phase shifter of 180.degree. (1/2.lambda.) and the other is not covered with anything as described in Japanese Patent Publication No. 50811 of 1987. According to the conventional phase shifting mask, a phase difference between the two adjacent apertures is 180.degree. (1/2.lambda.), so that contrast on a wafer is high. In the same manner, when three adjacent apertures are formed on the mask, a first aperture is covered with a phase shifter of 90.degree. (1/4.lambda.), a second aperture is covered with a phase shifter of 180.degree. (1/2.lambda.) and a third aperture is not covered with anything. According to the phase shifting mask, contrast on a wafer is generally high. However, contrast at medial areas between the first and second apertures and the first and third apertures is not sufficiently high, because each phase difference between them is 90.degree. (1/4.lambda.).
A phase shifting mask in which a plurality of small apertures covered with predetermined phase shifters, respectively, are formed around a main aperture is described in Japanese Patent Kokai No. 67514 of 1987. Further, a phase shifting mask in which a plurality of apertures are covered with phase shifters each having different shift amount is described on pages 491 and 492 of a preliminary report of "the 51 st applied physics institute lecture conference" of the autumn season, 1990 entitled "Fine Pattern Fabrication with Multistage Phase Shifter" and "A study of phase shifting mask construction applied for positive resist process" (JSAP Catalog Number: AP 901125-02). However, the phase shifting masks have a different object from the present invention.
Finally, it is noted that, a phase shifting mask in which a phase shifter having fine structure smaller than resolving power of the optical system is used as shading means instead of an opaque layer such as chromium is described on page 490 of the above preliminary report entitled "Transparent phase shift mask" (JSAP Catalog Number: AP 901125-02).